A parison is a manufactured article obtained by moulding of thermoplastic material and subjected to a blow moulding process which makes it into a bottle. Typically, the bottle obtained from the parison is intended for the bottling industry, for example to contain alimentary drinks and beverages.
The invention therefore addresses the bottling industry in particular and, more specifically, is applicable to the forming of containers from parisons made of thermoplastic material.
With reference in particular to the bottling industry, a parison is made of plastic material, generally transparent or semitransparent, having an elongate, tubular shape with an open upper part and a lower part constituting a closed bottom.
The parison is characterized by an upper part, called “mouth” or “neck”, which is not modified by the blow moulding process and thus remains unchanged in the finished container. The mouth also has on its outside surface a screw thread which differs in diameter and thread type, that is to say, the set of transversal ridges which are formed on its outside surface and which allow a cap to be screwed onto it.
Below the screw thread, the mouth generally has an annular region, called “flange”, which is a radially protruding ring.
The lower part of the parison has an elongate structure extending along an axis and tubular in shape. This is the part that is heated and blow moulded to form the container.
Parison moulding machines typically have a plurality of moulds in which the parisons are formed. Typically, a number identifying the mould which has been used to form the parison is stamped (in relief or low relief) on the outside surface of the threaded end of the parison itself.
In the container production process, any parison defects, especially on the threaded portion of the parison, can lead to considerable problems, such as, for example, jamming of the capping machine. This is particularly serious because bottling lines work continuously and their operating speeds (which translates as production capacity) are very high.
Parison inspection apparatuses are therefore known which are designed to check the quality of the parisons by optically analysing the threaded zone.
In this context, it is also useful to know the identification number of the mould used to produce a parison so as to facilitate identification of the cause of a defect and allow prompt action to be taken to eliminate the problem which caused the defect.
Some prior art apparatuses such as, for example, the one described in patent document EP1674234, involve handling the parisons in order to present them to an inspection camera under conditions which are favourable for inspection. In these apparatuses, inspection occurs off line because such handling of the parisons is incompatible with the speed at which the parisons move along the bottling line.
That means, typically, that the quality inspection is carried out only on a sample of the parisons made.
In these cases, typically, the cavity number (representing the mould in which the parison was produced) is derived from an image of an outside surface of the threaded end of the parison.
Another example of an optical inspection apparatus is provided by patent document JP5567079.
In that solution, a plurality of inspection stations operate in temporal succession on the same parison.
This prolongs the time needed for inspection and increases the overall dimensions of the apparatus.
In that solution, the cavity number of the parison is obtained by capturing images of the outside surface of the threaded end of the parison using a camera positioned laterally of the parison itself. This creates a problem of inspection precision if the image is captured with a camera which is stationary and the parison is moving at high speed. In other words, the system is precise only if the parisons are moved at a relatively low speed at the station where the cavity number is detected, which is incompatible with the need to keep in step with the bottling line.
In the solution proposed by JP5567079, there are also other inspection stations with cameras equipped with telecentric lenses designed to look inside the parisons in order to check the bottoms thereof. These stations, however, cannot be used effectively to analyse the screw thread and to read the cavity number.
Thus, the apparatus described in JP5567079, besides being cumbersome and constructionally complex, does not guarantee high performance.
Other examples of optical inspection devices are provided by patent documents DE102012022474A1 and WO2012/001414A2.
DE102012022474A1 describes (in FIGS. 7 and 8) the use of pericentric optical, to see from above the outer side walls of the inspected item (otherwise not visible for perspective reasons).
WO2012/001414A2 describes (FIG. 9) the possibility to top down viewing a preform with a camera 160; this solution has the function to inspect the neck and the bottom of the preform to verify that the colouring of the same is acceptable.
However, such solutions do not allow to read the number of the forming cavity or to effectively inspect the thread; in particular when the preforms move at high speed.